Reliability of rotating machinery, especially of above mentioned pumps, is defined on account of wear rate of the components and the bearing durability.
One of the important performance parameters for any centrifugal pump is its bearing life.
High pressure liquid in a pump exerts pressure on the outlet passages and shroud of the impeller, resulting in the generation of two forces, one in a lateral or radial direction and another in a longitudinal or axial direction with respect to the shaft axis.
The bearing life of a centrifugal pump depends upon the two hydraulic forces acting on the impeller, i. e. radial thrust and axial thrust.
Thrust balancing systems for centrifugal pumps are known in the art which have been devised to mitigate the effects of thrust on the bearings.
In a known balancing drum system, an axial load is transmitted to a balancing drum coupled to the pump shaft rotating in a static bush arrangement in the pump body cavity with a minimum radial clearance.
Fluid in the clearance forms a thin film that performs a useful bearing function, like a film of lubricant formed on a journal bearing.
In accordance with the known art, the radial clearance between the balancing drum and the bushing arrangement cannot be lowered below a set minimum value.
However, fluid leaking through the clearance amounts to a flow rate that may reduce the pump efficiency.
An axial thrust balancing system is known from U.S. Pat. No. 4,493,610.
Such axial thrust balancing system comprises a rotary shaft having an impeller mounted thereon. A sleeve is secured to the rotary shaft on the discharge side of the impeller for idle movement in an axial direction together with the rotary shaft.
A bush is fixedly attached to a casing enclosing the sleeve, juxtaposed against the sleeve with an annular clearance defined between the sleeve and the bush.
To solve the problem of too much fluid leaking through the annular clearance, the clearance is divided axially, by way of pressure chambers, into a plurality of shorter annular clearances formed between the sleeve and the bush that have the purpose of preventing an increase of the fluid flow rate.
Another axial thrust balancing system is known from U.S. Pat. No. 8,133,007.
The document discloses a multiple-stage centrifugal pump including a drum balancing device for eliminating unstable operations of the pump and reducing to a minimum the axial reactive force.
A leakage system is disclosed in the document making it possible to control and limit pump flow rate losses, thereby providing an improved efficiency pump.
The system is constituted by a controlled leakage hydraulic balancing drum coupled on a pump shaft and turning with a minimum radial clearance in a pump body or in a bush arrangement fixedly mounted on said pump body. A rotary ring element mounted on the balancing drum and rotatively driven therewith is arranged for only axial movement between the balancing drum and a fixed ring element mounted on said pump body. The rotary ring element has an end portion, forming with the fixed ring element a narrowing portion allowing the passage of leakage flow. Springs are provided which press the rotary ring element axially against the fixed ring element.
The balancing drum has a diameter such as to allow the control of the axial balancing force of the rotor of the pump. The rotary ring is hydraulically balanced so as to define a set leakage loss.
While the above arrangements may prove useful to provide leakage loss control through the clearance between the drum and the body of the pump, there is still a need for a simple and reliable leakage control system.